1. Field of the Invention
The present invention relates to an optical system capable of forming an image on a concave spherical surface. It especially applies to the case where the concave spherical surface comprises a pixel array capable of forming the detector of a digital image capture device, for example usable in a portable device.
2. Description of the Related Art
As illustrated in FIG. 1, when a lens 1 is used, parallel axial rays 3 form an image focused at a point F located on an image plane 5. However, if the field aperture is large, non-axial rays 7 cannot be considered to focus on a focal plane. Instead of this, these non-axial rays focus at points, such as point F′, located on a focusing surface 9 which generally is a sphere portion.
If it is desired for the image to effectively form on a planar focal surface, a complex optical system such as that described, for example, in U.S. Pat. No. 7,180,687, which comprises a lens comprising six complex aspherical lens surfaces for correcting irregular curvatures of the field, and the various aberrations and distortions inherent to an optical system.
An image capture system in which the image is formed on a concave spherical surface has been provided, for example, in U.S. Pat. No. 4,467,361. This system essentially comprises a ball lens. A disadvantage of such ball lenses, in addition to the fact that they provide a poor correction of distortions, is that their manufacturing is incompatible with collective methods developed to manufacture lenses by deposition of a layer on a transparent plate, and simultaneous forming of a large number of identical lenses by molding, assembly, and cutting into individual lenses. Such collective methods are for example described in a document entitled “wafer-level cameras” by Margaret Zoberbier et al. disclosed at the International Image Sensor Workshop, Bergen, Norway 2009.
U.S. Pat. No. 7,688,531 describes a system for forming images on a planar surface and essentially aims at performing astigmatism corrections, but does not concern field curvature aberrations.
Thus, known systems have various disadvantages.